Many of today's high precision fabrication process are conducted in vacuum chambers. These processes include vacuum deposition of material, optical and electronic lithography mask pattern transference in semiconductor layer formation by selective removal of materials, Micro-Electro-Mechanical Systems (MEMs), Lab-on-a-Chip (LOC) Devices, Nano-Electro-Mechanical Systems (NEMS), micro scale controlled growth of materials, and the likes. Working in a vacuum environment is complicated by relatively nonexistent heat transfer rates through vacuum, by the high volatility of many otherwise stable substances in vacuum, and by similar thermodynamic extreme cases that arise in vacuum. These problems may be circumvented for applications requiring introducing mechanical motion into an appendage of the vacuum chamber by using complex systems of transfer rods inside flexible bellows, which link a source of mechanical power located outside the chamber with a work-piece manipulation requirement located in the chamber. Such an arrangement is described as a prior art in U.S. Pat. No. 5,784,925 (FIG. 1). In this prior art heat generation and vacuum contamination risks are reduced considerably, since the motors that produce the power for the motion are outside the vacuum chamber. However, the transfer rods and the flexible bellows pose a practical limitation on the rigidity of the system, and hence its accuracy and resistance to vibrations, and on the extent of the motion of the work piece that it enables.
U.S. Pat. No. 5,784,925 teaches yet another solution for providing accurate linear motion inside a vacuum chamber. In U.S. Pat. No. 5,784,925 a fluid bearing is incorporated inside a vacuum chamber characterized by a first pressure, and a fluid disposal system maintains a region with a second pressure level, higher than the first pressure but substantially lower than one atmosphere. Motors inside the vacuum chamber provide the motion to collars that slide on rods through the said air bearings and carry the work piece.
Albeit the advantages in providing accurate motion of a work piece inside a vacuum chamber, substantially within the size of similar systems that operate in atmosphere, U.S. Pat. No. 5,784,925 still suffers a few drawbacks. Sagging of the rods under the payload weight degrade the flatness of the motion. The motors inside the vacuum chamber are a source of heat, contamination and electromagnetic fields, all being a threat to the process taking place inside the vacuum chamber, or adding complexity to the system design. Also, flexible pipes that carry the fluid to the fluid bearing inside the vacuum chamber, and then remove it from the bearing outside of the vacuum chamber, must be carried with the moving collars that slide on the rods, thus degrading the smoothness of the motion provided by the system.
Substantially any accurate industrial fabrication processes which take place in vacuum, including all of the aforementioned micro scale processes, share a mutual need for high spatial precision. Particularly, there is an ongoing need to improve the precision of work-piece stage motion, to improve the accuracy of measurement with respect to the location and orientation of any predetermined point on the work-piece, and, in conjunction with the aforesaid, to improve any work-piece deflection variability, which ordinarily complicates precision motion or normalization of precision measurements.
Basic approaches to progressive improvements for these kinds of micro-scale fabrication work-piece based systems have been developed. For example, U.S. Pat. No. 6,126,169 presents numerous configurations for using an air-bearing seal to facilitate work on a work-piece stage. U.S. Pat. No. 6,163,033 teaches the use of an air-bearing seal by which a vacuum chamber can be laterally slid over a polished work-piece without perturbing the quality of the chamber's vacuum.
The U.S. Pat. No. 6,163,033 technology desirably provides that substantially all motors, guides and bearings may be outside of the vacuum chamber, thereby allowing a reduction of the dimensions of the vacuum chamber to, for example, the work-piece size plus the required motion range. Since no heat is generated inside the vacuum chamber by these external motors, there is a resultant increased thermal stability. An additional benefit arises in that distancing motors from the work-piece distances electromagnetic fields sources, which may disturb fabrication processes such as electron beam lithography, charged particle etching, or ion deposition
Also, in U.S. Pat. No. 6,163,033 the risk of out-gassing, the complexity of adapting motors to a high vacuum environment, the need for heat sinking, and the need for EMF shielding are selectively avoided. Likewise, fewer wires, gas-carrying pipes or other moving parts are connected to the stage inside the vacuum chamber or associated with it. Thus out-gassing is comparatively minimized.
Nevertheless, substantial motion accuracy is not exactly facilitated in U.S. Pat. No. 6,163,033 since new work-piece deflections are introduced. Embodiments of U.S. Pat. No. 6,163,033 suffer from resultant work-piece deflection variability, and from introduced complications effecting work-piece location precision.
Simply stated, there are two fundamental shortcomings that are inadvertently introduced with embodiments of U.S. Pat. No. 6,163,033. Firstly, deflections of the flatness of the work-piece occur because of stress imposed by the air-bearing seals to selected regions of the work-piece where the air-bearing seals (and the vacuum chamber circumscribed thereby) interface thereto. These deflections are further complicated in that they migrate with the laterally sliding of the vacuum chamber. Secondly, access limitations to the desired fabrication process location of the work-piece are imposed by the considerable structural integrity requirements of the work-piece per se; since the common wisdom solution to work-piece deflections has been to use a polished heavy monolithic slab for the work-piece.
Nevertheless, there is a need in the art for further improvements to these aforementioned embodiments of the U.S. Pat. No. 6,163,033 type systems, or the likes. Basically, it would be a significant improvement in these type systems if the deflection perturbations of the work-piece flatness could be managed in a predetermined way, and preferably reduced. For many applications, it would also be considered an improvement in the art if the stage design could comply with an opening or relatively thin or transparent surface in its central work area, even if aspects of the work-piece deflection problem remained; since such access is presently cumbersome in light of the massive work-pieces currently in use.
While these needs in the art can be simply described, one should remain cognizant to maintaining known work-piece improvements with respect to minimizing out-gassing, distancing sources of EMF, and avoiding introducing of unnecessary heat sources with the vacuum.